1. Technical Field
Displays are broadly based on emissive or modulation technologies. The former create light at each pixel location, whereas the latter modulate light that flood illuminates the display panel. This patent disclosure generally relates to liquid crystal displays that modulate light that flood illuminate the display panel, and more specifically this disclosure relates to controlling the spatial and temporal characteristics of the illumination to such panels.
2. Background
Most LCD displays are illuminated with uniform static white light from a back light unit (BLU). Most BLUs have as their light source, cold cathode fluorescent lamps (CCFLs) that edge light diffusing waveguide sheets. The resultant light is emitted from the large area surface of the waveguide with good (though static) spatial, spectral, and angle uniformity. Images are then displayed by modulating the intensity of the incident light at individual pixel locations. Color is created by filtering out two of the three RGB primary color components of the white light at each pixel. Independent modulation of the remaining primary colors provides full color displays. The approach is very inefficient since two thirds of the available white light is absorbed in these filters and is wasted.
More recently, LEDs have been used to illuminate LCD panels, as they offer advantageous features such as long lifetime, instant on, mercury free etc. On the downside, these devices are more localized than the distributed CCFL and are inherently colored. Using them to illuminate large area displays with white light is more challenging, often resulting in unacceptable color uniformity. Advanced techniques are often used to avoid this using feedback with optical sensing. With sufficient control, LEDs can enhance displays through local area illumination, where different areas of the display are illuminated nearly independently. This is how scrolling and spatial dimming are implemented. The former reduces motion blur and allows for faster panel update, whereas the latter leads to increased effective bit depth in high contrast images. Both these approaches have been implemented in commercial products (e.g., Model LCD LN-T4681F by Samsung).
A further sophistication in local illumination is described in the commonly-assigned patent application Ser. No. 11/944,583, entitled “Illumination Systems for Visual Displays,” filed Nov. 23, 2007, herein incorporated by reference. In this patent application, laser-like illumination sources provide local illumination on a pixel level. This approach, which can also be achieved with bright LEDs, provides separate red, green and blue illumination to panel pixels, removing the need for pixel color filtering—the so-called ‘color filter array’ or CFA. In this passive form, it yields not only much higher efficiencies than the incumbent approach, but also reduces panel costs through removal of the CFA.
A demand on resolution of LC panels occurs in autostereoscopic LCD displays. This allows the provision of 3D images to a viewer without any eyewear. Conventionally, autostereoscopic technology works by diverting each column of pixels to an individual eye, essentially halving the vertical resolution but providing a unique image to each eye. By displaying a correctly rendered image on the monitor it is then possible to trick the brain into believing that the two 2D images are a single 3D scene.
There are generally two ways of directing the light to each eye, either by using a parallax barrier or a lenticular lens. The two diagrams below illustrate how the light from “left” and “right” pixel columns reach the eye. FIG. 14 illustrates the parallax barrier technique, in which a mask or parallax barrier is placed over the LCD display which directs light from alternate pixel columns to each eye. Parallax barrier displays allow instant switching between 2D and 3D modes as the light barrier is constructed from a layer of liquid crystal which can become completely transparent when current is passed across, allowing the LCD to function as a conventional 2D display.
FIG. 15 illustrates the lenticular lens technique, in which an array of cylindrical lenses directs light from alternate pixel columns to a defined viewing zone, allowing each eye to receive a different image at an optimum distance. FIG. 16 illustrates that both the parallax barrier and the lenticular lens technique produce zones in which the viewer can see a 3D image and the head must be correctly aligned within these zones to comfortably view the scene. If the head is located outside of these zones then both image streams will reach each eye or the viewing positions will be inverted, both of which will cause comfort problems.
Owing to the splitting of the horizontal resolution between two images, a shortcoming of conventional LC panel autostereoscopic displays is that the horizontal resolution of the LC screen is effectively halved when displaying 3D images because half of the pixels are devoted to displaying the right eye image, while the other half of the pixels are devoted to the left eye image.